On Thursday, Nike unveiled a series of startups that it has allowed to tap directly into data about users of its Nike FuelBand device, which tracks activity like running and walking. One of the most intriguing ideas from the companies: advertising based on your physical activity.

Boston-based HighFive bills itself as a mobile ad network based on real-life activity. HighFive plugs into existing apps, like cross-fit tracker myWOD, to surprise consumers with offers. So far, they’ve signed up brands such as Ellie, a yoga gear maker, who hope the offers lure new customers.

The FuelBand partnership offers a glimpse of a future of wearable, sensor-based advertising. Today, digital marketers target consumers based on their Web browsing habits, or the personal information they share with services like Facebook. If wearable computers like the FuelBand and Google Glass take off, ad targeting could get even more intimate, based on our location, body movements – even heart rate.

“There is a fine line between serving rewards that are awesome versus creepy,” said HighFive COO Katie Pietrowski. Participating in the service – and sharing data from your FuelBand — is voluntary, she noted. Ideally, she said, the company wants it to feel like users happen to be running past a local fitness shop where an employee standing in front gives them a high-five and hands them a coupon.

Another startup that’s part of the Nike program, Sprout, gives users the chance to plug their FuelBand fitness data into a wellness program run by their employer. If an individual employee or a team in the office does particularly well, the company could offer them rewards. Big companies who pay for employee health insurance are looking for ways to inspire employees to get fit, Sprout’s founders say.

Nike itself has been very cautious about using the screen on the Fuelband device — or the data it collects — for advertising. While HighFive uses FuelBand data, Nike so far isn’t part of the startup’s ad network.

“Technically, there’s no reason why we couldn’t create the capability so that when you walk into a store, we’ll know you’re there and can give you better service,” said Stefan Olander, Nike’s head of digital sport in a recent interview. “But we have to make sure it feels like a service rather than stalking.”

That doesn’t mean Nike can’t see doing it in the future. “I don’t think people have problems giving up personal data as long as they know exactly why it’s used and that its to their benefit,” Olander said. “But you have to be careful with privacy, as those things get a little creepy.”

For the time being, Nike says it uses the data it collects to help it design new products and craft merchandising strategies. For instance, when the company saw that more people reported on its Nike Plus system that they were running on trails, rather than on paved roads, the company increased the variety of trail-running merchandise.